Through DE-C2-33 37 715 a directional attitude reference device is known, in which a two-axis position gyro having a substantially horizontal spin axis is arranged on an inner gimbal. The position gyro has position pick-offs and torquers on the input axes. The inner gimbal is mounted in an azimuth gimbal about a substantially horizontal gimbal axis. The azimuth gimbal is, in turn, mounted rotatably on a base about a substantially vertical gimbal axis, the azimuth axis. The two gimbal axes are mutually orthogonal. The gimbal axis between inner gimbal and azimuth gimbal is perpendicular to the spin axis of the gyro. A first input axis of the gyro is parallel to the gimbal axis of the inner gimbal and perpendicular to the spin axis. The second input axis of the gyro is perpendicular to the first input axis and to the spin axis.
A level sensor responds to inclination of the inner gimbal relative to the horizontal about the first input axis.
A servo electronic system having pick-offs is provided at the gyro. The signals of this servo electronic system energize the servomotors such that the inner gimbal with a gyro housing is electrically restrained to the spin axis. By a first control loop, the measuring sensor of which is the level sensor, the spin axis of the gyro is always maintained horizontal. To this end the level sensor energizes a torquer acting about the second input axis. This torquer effects a deflection of the gyro about the first input axis. By means of the servo electronic system the position pick-off acting about the first input axis actuates, in turn, the inner gimbal such that the spin axis is maintained horizontal.
Furthermore, a second control loop is provided, the controlled quantity of which is the torque effective about the second input axis. This second control loop energizes the torquer acting about the first input axis, such that the azimuth gimbal is rotated through the servo electronic system into a position in which the angular rate effective about the first input axis becomes zero.
With the positional attitude reference device according to said DE-C2-33 37 715 a continuous deflection of the position gyro and a corresponding inclination of the inner gimbal is effected, when an angle between the spin axis of the gyro and North is present. The level sensor responds to this inclination. The signal from the level sensor is applied to the first control loop, an aligning controller. The output signal from this first control loop is applied as controlling signal to a torquer of the gyro and causes deflection of the gyro. This deflection effects, in turn, alignment of the spin axis with North through the corresponding pick-off and the servomotor controlled thereby and acting in azimuth.
The controlling signal applied to the torquer in the first control loop in the balanced state is stored. The servomotor acting in azimuth is adapted to be energized such that the inner gimbal subsequently is rotated in azimuth by a fixed angle. From the stored signal and the controlling signal obtained after the rotation, a signal is generated through a computer. This signal corresponds to the then present angle between spin axis and North. Subsequently, the servomotor acting about the azimuth axis is energized according to this signal such that the spin axis is aligned with North.
In the arrangement according to DE-C2 33 37 715 the problem is pre-aligning the spin axis of the gyro with North with a directional attitude reference device by using available components. The problem is not compensation for gyro errors.
A device for determination of North direction by means of a gyro influenced by the rotation of the earth is known through DE-B1-29 03 282. This gyro is two-axis. The spin axis of the gyro is vertical. Position pick-offs and torquers are arranged at two input axes perpendicular to each other and to the spin axis. The signal of each position pick-off associated with one input axis is applied to the torquer associated with the other axis, whereby the gyro is electrically restrained to the vertical with its spin axis. Through a North deviation computer the angle between one of the input axes and North direction is computed from the two signals which are applied to the torquers and which correspond to the components of the horizontal component of the rotary speed of the earth.
In DE-B1-29 03 282 the two signals are stored in a memory. Subsequently the gyro is rotated by 180.degree. about its spin axis. In this rotated position the signals applied to the torquers are stored again. Then, for computing the North deviation, the sums and the differences of the associated stored signals are formed. Gyro errors are eliminated by this sum and difference formation.
A device for determination of North direction is known from DE-C2-30 50 615. This device comprises an azimuth gimbal which is rotatably mounted about an azimuth axis. A rate gyro is arranged at the azimuth gimbal, which rate gyro responds to components of the rotary speed of the earth and supplies corresponding signals. The spin axis of the rate gyro is located in a plane perpendicular to the azimuth axis. An input axis of the rate gyro extends in this plane perpendicular to the spin axis. The azimuth gimbal is rotatable by a servomotor into a 0.degree.-position, a 90.degree.-position, a 180.degree.-position and a 270.degree.-position. The signals obtained from the rate gyro are stored. North direction is determined by a computer from the stored signals. Therewith, gyro errors due to gyro drifts are compensated for by forming differences. Advantage is taken of the fact that, when the gyro is rotated by 180.degree. the signals caused by the rotary speed of the earth change sign, that means that they are added when forming differences, while most of the gyro errors keep their sign after the rotation and thus are eliminated by the forming of differences.
EP-A1-0 048 212 describes a directional attitude reference device having a two-axis position gyro having the spin axis substantially horizontal on an inner gimbal. The inner gimbal is rotatably mounted in an azimuth gimbal about a pivot axis parallel to the first input axis of the position gyro. Two level sensors are located on the inner gimbal, two other level sensors are arranged housing-fixed. An angle sensor on the azimuth axis supplies an azimuth signal. The inner gimbal is decoupled from the movement of the support about two axis by servomotors which are controlled by pick-offs of the position gyro. The movement about the third axis is measured by a first level sensor and taken into account in the computer. Signals for compensation for the vertical component of the rotation of the earth are applied to torquers which act about the input axes on the position gyro. By applying the signal from the second level sensor to the torquers on the input axes of the position gyro this second level sensor is moved into horizontal position and the spin axis of the gyro is rotated into North direction.
US-A-4 458 426 shows a gyro compass having a gimbal which is rotatable about an upright axis and a gyro which is rotatably mounted in this gimbal about an inner axis extending crosswise to the upright axis. The gyro compass also comprises a level sensor which is located on the gimbal and supplies a level signal which corresponds to the tilting of the upright axis out of the vertical. A controller is connected to the level sensor. The controller responds to the level signal and rotates the gimbal into a initial position in which the level signal becomes zero. The gimbal is rotated until the level sensor indicates that the inner axis is horizontal. Then the torque is measured, which is exerted on the gyro by the horizontal component of the rotation of the earth when the spin axis of the gyro is rotated about the inner axis into a horizontal position. Subsequently, the gimbal is rotated by 180.degree. and the torque is measured again, which is exerted on the gyro by the horizontal component of the rotation of the earth when the spin axis is brought again into a horizontal position about the inner axis.